We conceive of the natural world as populated by relatively persistent material things standing in spatio-temporal relations to each other. They come into existence, exist for a time, and then pass away. We locate them relative to landmarks and to other material things in the landscape which they, and we, inhabit. We characterize them as things of a certain kind, and identify and re-identify them accordingly. The expressions we typically use to do so are, in the technical terminology derived from Aristotle, names of substances.1 The term ‘substance’ has two distinct, but importantly linked, meanings. In the Aristotelian sense, a substance (more accurately, ‘a primary substance’) is a concrete individual thing of a given kind, such as a particular human being (Socrates), a given tree (such as Gautama’s Bo-tree) or a certain stone (the Kohinoor). The general kind (the ‘secondary substance’ in Aristotle’s terminology) to which the individual substance belongs is specified by a substance-name (‘human being’, ‘pipal tree’, ‘diamond’). Individual substances are the basic objects of reference and subjects of predication in our conceptual scheme. They are things of one kind or another (specified by a substance-name, as when we say that Socrates is a man). They are qualified by numerous properties, specified by non-substantial predicates (for example, is in the agora, is snub-nosed, or is a philosopher), but they are not themselves predicable of things – Socrates cannot be said to qualify anything or to be true of anything (as opposed to being like Socrates, which is a relational property some rare people may have) . Nor can the proper name ‘Socrates’ be said to be true of anything, by contrast with the identifying phrase ‘is Socrates’, which is true of the teacher of Plato, and tells us who he is. Characterizing an individual as a thing of a given kind by using such a (secondary) substance-name answers the question of what the thing is. Grasp of the substance name implies knowledge of what being a such-and-such consists in, in so far as that is logically (or, in the extended sense of the term, grammatically) determined..